1. Field of the Invention
The invention relates to an optical fiber having a core whose material consists essentially of vitreous silica, and having a jacket of plastic whose index of refraction n.sub.jacket is smaller than that of the core, and which is optically active at least on a length of 10 meters reckoned from the light-input end of the fiber.
2. Discussion of Prior Art
Optical fibers of the kind described above are known from U.S. Pat. No. 3,869,194. In the comparison to optical fibers whose core and jacket are of vitreous material, plastic-jacketed optical fibers have the advantage that they are less expensive to manufacture. The plastic-jacketed glass core optical fibers have a high numerical aperture in comparison to optical fibers having core and jacket of vitreous material.
At the interface between the core of vitreous material and the plastic jacket there occurs a total reflection of the light carried in the core of the fiber. The bonding together of the core and jacket material requires a maximum of care, and yet scatter losses are unavoidable at the interface between core and jacket on account of flaws, dust particles and the like.
Consequently, it has been proposed to reduce the scatter losses by replacing glass-core plastic-jacketed optical fibers with optical fibers consisting only of a core material. For this purpose, according to German Auslegeschrift No. 1,901,053, the outer layer of the core material has been modified by a diffusion process so that it acts as a jacket. In that case, however, the optical fiber is the same as those made only of vitreous materials having low numerical aperture.
It is the object of the invention to provide an optical fiber which, while retaining a high numerical aperture, provides better light guiding properties, i.e. having lower losses, than the known plastic-jacketed optical fibers, while at the same time retaining the advantage of low-cost manufacture.